A digital coherent optical receiving system is a technology of performing coherent optical reception of signal light which is transmitted with a signal being applied to the amplitude or phase of an optical electric-field at a transmitting side, converting an optical signal obtained by performing the coherent optical reception into an electric signal, and regenerating an original signal by performing digital signal processing on the electric signal. In the digital coherent optical receiving system, it can be realized with a high degree of accuracy by the digital signal processing to perform waveform equalization processing such as chromatic dispersion compensation or the like, which becomes a large problem at the time of long-haul transmission by an optical fiber system.
The coherent optical reception includes the steps of mixing the inputted signal light with local light (local oscillation light) which has almost the same frequency as the signal light by inputting them into a 90-degree optical hybrid circuit, and obtaining as an output an interference light with down-converted frequency which is generated by the mixing. The output from the 90-degree optical hybrid circuit includes a set of two lights (in-phase component and quadrature component) whose phases differ from each other with 90 degrees, which are photoelectrically converted in a photoelectric conversion circuit and outputted as an electric signal which represents an electric field envelope of the signal light. This electric signal is converted into a digital signal, on which the waveform equalization processing is performed by the digital signal processing.
Examples of such a coherent optical receiver are disclosed in Japanese Patent Application Laid-Open Publication No. 2008-153863 and Japanese Patent Application Laid-Open Publication No. 07-283793.
Japanese Patent Application Laid-Open Publication No. 2008-153863 discloses a technology relating to an optical receiver of the coherent reception system that can receive signal light of high bit rate independent of a polarization state of the signal light. According to the technology disclosed by Japanese Patent Application Laid-Open Publication No. 2008-153863, the local oscillation light having orthogonal polarization components whose optical frequencies differ from each other is mixed with the received signal light, and then the mixed wave signal is converted into an electric signal in a photoelectric conversion unit. As a result, each intermediate frequency signal arises which is generated by a beat of each orthogonal polarization component and a received optical signal. The difference of the optical frequency between these orthogonal polarization components is set so as to be smaller than twice the bandwidth of the received signal light and not to become larger than a spectral line width of a light source generating the received signal light and a spectral line width of a light source generating the local oscillation light. Therefore, each electric spectrum of the intermediate frequency signal becomes overlapped, and it is possible to narrow the bandwidth required in an electronic circuit used in the subsequent stage to about twice the bandwidth of the signal.
Japanese Patent Application Laid-Open Publication No. 07-283793 discloses a technology for solving a problem that a bit rate of signal light cannot be increased to the same extent as the band of a light receiving element, and performing the coherent optical reception of an optical signal modulated by means of the optical FSK (frequency shift keying) modulation system as a narrowband signal. According to the technology disclosed in Japanese Patent Application Laid-Open Publication No. 07-283793, an optical phase locked loop is formed which controls the optical phase of a local oscillation optical signal by a phase error signal, and the optical frequency of a local oscillation light source is kept situated between the frequency corresponding to a mark code and the frequency corresponding to a space code of the optical FSK signal. And, a bit judgment circuit determines whether the phase of the optical FSK signal is advanced or delayed compared with the local oscillation optical signal and determines a mark and a space of the received signal.